Synthesis of di- and trinuclear zinc complexes bearing pyrrole-based ligands and their catalysis toward the ROP of rac-lactides

Di(aminomethylpyrrolyl)methane derivatives [2-{5-R′ 2 NCH 2 (C 4 H 2 NH)}] 2 CR 2 ( 2a R = Et, R′ = Me; 2b R = Ph, R′ = Me; 2c R = Ph, R′ = Et; 2d R 2  = 1,1′-biphenyl-2,2′-diyl, R′ = Me; 2e R 2  = 1,1′-biphenyl-2,2′-diyl, R′ 2  = (CH 2 ) 4 ) were prepared through reaction of di(1 H -pyrrol-2-yl)methane derivatives (2-C 4 H 3 NH) 2 CR 2 ( 1a R = Et; 1b R = Ph; 1c R 2  = 1,1′-biphenyl-2,2′-diyl) with formaldehyde and amines. Treatment of 2a with 3 equiv. of ZnEt 2 in toluene afforded trinuclear zinc conplex 3a , while respective reaction of 2b - 2e with 3 equiv. of ZnEt 2 in toluene or THF produced dinuclear zinc complexes 3b - 3e . Each of the complexes was characterized by 1 H and 13 C NMR spectroscopy and elemental analysis . Structures of complexes 3a and 3b were additionally characterized by single crystal X-ray diffraction. Catalysis of complexes 3a - 3e toward the ROP of rac -lactides was evaluated in both toluene and THF. Each complex was active in the catalysis in the presence of benzyl alcohol (BnOH). 3a showed the highest activity in toluene and 3c was the most active in THF. Kinetic study of complexes 3a and 3e was carried out and the results showed that the polymerization catalyzed by 3e /BnOH follows a first-order kinetics, while 3a /BnOH-catalyzed polymerization followed neither first-order nor second-order kinetics.